Function dynamic_loggers_in_use 

pub fn dynamic_loggers_in_use() -> usize

How many of the dynamic_logger_capacity slots are spent.

Counts slots CLAIMED, which is one more than the loggers reachable by name whenever two threads raced the same new name: the loser’s slot is spent and unreferenced. Bounded and reported rather than reclaimed — this arena has no free list by construction.